You can check in, and should online at T-24 to get the best boarding position, unless you buy Early Bird.

But to get your boarding pass, you will have to check in on a Kiosk, or the check-in desk once you arrive at your airport. This has to be done at least 1 hour prior to your flight. This is where they check to make sure your passport info matches your ticket.

Also they will ask you to hold your passport in your hand as you board the plane, you dont have to open it, they just are verifying everyone makes it onto the plane with a passport, so you dont get stuck in immigration in Mexico without a passport.